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Arithmetic expansion provides a powerful tool for performing (integer) arithmetic operations in scripts. Translating a string into a numerical expression is relatively straightforward using backticks, double parentheses, or let.
- Arithmetic expansion with backticks (often used in conjunction with expr)
z=`expr $z + 3` # The 'expr' command performs the expansion.
- Arithmetic expansion with double parentheses, and using let
The use of backticks (backquotes) in arithmetic expansion has been superseded by double parentheses --
$((...))-- and also by the very convenient let construction.
z=$(($z+3)) z=$((z+3)) # Also correct. # Within double parentheses, #+ parameter dereferencing #+ is optional. # $((EXPRESSION)) is arithmetic expansion. # Not to be confused with #+ command substitution. # You may also use operations within double parentheses without assignment. n=0 echo "n = $n" # n = 0 (( n += 1 )) # Increment. # (( $n += 1 )) is incorrect! echo "n = $n" # n = 1 let z=z+3 let "z += 3" # Quotes permit the use of spaces in variable assignment. # The 'let' operator actually performs arithmetic evaluation, #+ rather than expansion.
Examples of arithmetic expansion in scripts: